The present invention relates to a way of making an organic vertical cavity laser light producing device.
Vertical cavity surface emitting lasers (VCSELs) based on inorganic semiconductors (e.g. AlGaAs) have been developed since the mid-80""s (K. Kinoshita et al., IEEE J. Quant. Electron. QE-23, 882 [1987]). They have reached the point where AlGaAs-based VCSELs emitting at 850 nm are manufactured by a number of companies and have lifetimes beyond 100 years (K. D. Choquette et al., Proc. IEEE 85, 1730 [1997]). With the success of these near-infrared lasers in recent years, attention has turned to other inorganic material systems to produce VCSELs emitting in the visible wavelength range (C. Wilmsen et al., Vertical-Cavity Surface-Emitting Lasers, Cambridge University Press, Cambridge, 2001). There are many fruitful applications for visible lasers, such as, display, optical storage reading/writing, laser printing, and short-haul telecommunications employing plastic optical fibers (T. Ishigure et al., Electron. Lett. 31, 467 [1995]). In spite of the worldwide efforts of many industrial and academic laboratories, much work remains to be done to create viable laser diodes (either edge emitters or VCSELs) which span the visible spectrum.
In the effort to produce visible wavelength VCSELs, it would be advantageous to abandon inorganic-based systems and focus on organic-based laser systems, since organic-based gain materials can enjoy the properties of low unpumped scattering/absorption losses and high quantum efficiencies. In comparison to inorganic laser systems, organic lasers are relatively inexpensive to manufacture, can be made to emit over the entire visible range, can be scaled to arbitrary size, and most importantly, are able to emit multiple wavelengths (such as red, green, and blue) from a single chip.
The usual route for making a manufacturable laser diode system is to use electrical injection rather than optical pumping to create the necessary population inversion in the active region of the device. This is the case for inorganic systems since their optically pumped thresholds (P. L. Gourley et al., Appl. Phys. Lett. 54, 1209 [1989]) for broad-area devices are on the order of 104 W/cm2. Such high power densities can only be achieved by using other lasers as the pump sources, precluding that route for inorganic laser cavities. Unpumped organic laser systems have greatly reduced combined scattering and absorption losses (xcx9c0.5 cmxe2x88x921) at the lasing wavelength, especially when one employs a host-dopant combination as the active media. As a result, optically pumped power density thresholds below 1 W/cm2 should be attainable, especially when a VCSEL-based microcavity design is used in order to minimize the active volume (which results in lower thresholds). The importance of power density thresholds below 1 W/cm2 is that it becomes possible to optically pump the laser cavities with inexpensive, off-the-shelf, incoherent LED""s.
In order to produce single-mode (or a few modes) milliwatt output power from an organic VCSEL device, typically it is necessary to have the diameter of the emitting area be on the order of 10 xcexcm. As a result, 1 mW of output power would require that the device be optically pumped by a source producing xcx9c6000 W/cm2 (assuming a 25% power conversion efficiency). This power density level (and pixel size) is far beyond the capabilities of LED""s and, additionally, would most likely cause some degradation problems with the organic materials if they were driven cw. A path around that problem is to increase the organic laser""s emitting area diameter to around 350 xcexcm, which would reduce the pump power density level to 4 W/cm2 (to produce 1 mW of output power). This power density level and pixel size is achievable by off-the-shelf 400 nm inorganic LED""s. Unfortunately, broad-area laser devices having 350 xcexcm diameter emitting areas would lead to highly multimode output and to lower power conversion efficiencies (as a result of filamentation). As a result, it is highly advantageous to produce large area organic VCSEL devices, which have good power conversion efficiencies and single-mode (or a few modes) output.
It is an object of this invention to provide a way of making an organic surface emitting laser arrangement that is particularly suitable to permit laser emission from a two-dimensional array of micron-sized organic laser pixels.
These objects are achieved by a method of making an organic vertical cavity laser array device, comprising:
a) providing a substrate;
b) providing a first portion of a bottom dielectric stack reflective to light over a predetermined range of wavelengths and being disposed over the substrate;
c) forming an etched region in the top surface of the first portion of the bottom dielectric stack to define an array of spaced laser pixels which have higher reflectance than the interpixel regions so that the array emits laser light;
d) forming a second portion of the bottom dielectric stack over the etched first portion;
e) forming an organic active region over the second portion of the bottom dielectric stack for producing laser light; and
f) forming a top dielectric stack over the active region and spaced from the bottom dielectric stack and reflective to light over a predetermined range of wavelengths.
It is an advantage of the present invention to provide a way of making a two-dimensional organic laser array device employing micron-sized laser pixels which can be optically driven by large area sources and produce either single or multi-mode laser output, such that there is reduced scattering loss and increased power conversion efficiency. The devices employ a microcavity design incorporating high reflectance dielectric stacks for both the top and bottom reflectors; and have a gain media including organic material. The micron-sized laser pixels of the device are provided by selectively modulating the reflectance of the bottom dielectric stack. The modulation of the reflectance is obtained via a multistep process including growing the first portion of the bottom dielectric stack, etching micron-sized laser pixel into the surface of the first portion, and then depositing the second portion of the bottom dielectric stack over the etched surface. As a result of burying the etched surface deep within the bottom dielectric stack (and away from the gain region), there is reduced scattering loss and increased power conversion efficiency for two-dimensional organic laser array devices.